R1:49d93dcf98aa

Add a `cluster.read-only` option

Summary:
Ref T4571. There will be a very long path beyond this, but add a basic read-only mode. You can explicitly enable this to put Phabricator in a sort of "maintenance" mode today if you're swapping databases or something.

In the long term, we'll automatically degrade into this mode if the master database is down.

Test Plan:
- Enabled read-only mode.
- Browsed around.
- Didn't immediately see anything that was totally 100% broken.

Most stuff is 80-90% broken…